Doors are used in factories, warehouses, stores, garages, etc., to provide isolation against cold or noise, either from the outside or between adjacent premises. Such doors generally comprise a frame which in raisable doors, is constituted by two side uprights interconnected at their top ends by a horizontal cross-member, together with a curtain that is generally made of plastic and that is suitable for being raised quickly to open a passage through the door and which is collected together at the top of the door above the passage opened therethrough, either by being rolled up or else by being folded up concertina-like. The invention relates more particularly to doors that are transparent, at least in part. A vehicle, such as a truck or a handling carriage, etc., must be capable of passing through the door quickly. When a vehicle approaches, the curtain is automatically raised vertically or is automatically moved away sideways. Thus the vehicle can maintain speed as it goes through the door it is important that the driver of the vehicle see whether the space on the other side of the door is empty or whether another vehicle is arriving from the opposite direction.
When a curtain having a transparent portion is collected together at the top of the door or to the side thereof, the various different portions of the curtain can rub against one another. The surface of the curtain which is generally made of plastic is thus roughened rapidly, thereby reducing visibility therethrough.
Curtains are often reinforced by horizontal stiffening bars which serve to guide the curtain in slideways at the sides of the curtain. While a curtain is being wound up, such bars rub against the transparent portions of the curtain, thereby worsening the roughening effect. In addition, while a curtain is being wound up or down, whenever one of its bars engages already-wound portion of the curtain, or whenever it leaves that portion, considerable noise is generated which is detrimental to the surroundings.